Hyperthyroidism, often referred to as an 'overactive thyroid', is a condition in which the thyroid gland produces and secretes excessive amounts of the free (not protein bound and circulating in the blood) thyroid hormones, triiodothyronine (T3) and/or thyroxine (T4). This is the opposite of hypothyroidism ('sluggish thyroid'), which is the reduced production and secretion of T3 and/or T4. Hyperthyroidism is a type of thyrotoxicosis, a hypermetabolic clinical syndrome which occurs when there are elevated serum levels of T3 and/or T4. Graves disease is the most common form of hyperthyroidism. While hyperthyroidism may cause thyrotoxicosis they are not synonymous medical conditions; some patients may develop thyrotoxicosis as a result of inflammation of the thyroid gland (thyroiditis), which may cause the release of excessive thyroid hormone already stored in the gland but does not cause accelerated hormone production.
Contributions by Davidruben, 220.127.116.11, and Arcadian.